2.1. Liposomes
Liposomes are lipid vesicles which can entrap a variety of pharmaceutical agents and can be used for delivery of these agents to cells and tissues in vivo. A multitude of liposomes can be constructed from one or more lipids such that they are small unilamellar vesicles (SUV), large unilamellar vesicles (LUV), oligolamellar vesicles prepared by reverse phase evaporation (REV), or multilamellar vesicles (MLV). See review by Deamer and Uster, 1983, "Liposome Preparation: Methods and Mechanisms," 1983, in Liposomes, Ostro, ed., Marcel Dekker, Inc., New York, pp. 27-51.
Much has been written regarding the possibilities of using liposomes for drug delivery systems. See for example, the disclosures in U.S. Pat. No. 3,993,754, U.S. Pat. No. 4,145,410. In liposome delivery systems the medicament is entrapped in the liposome which is administered to the patient to be treated. See U.S. Pat. No. 4,224,179 and U.S. Pat. No. 4,235,871.
Aqueous suspensions of liposomes may be inoculated in any desired way (e.g., intravenously, intramuscularly, intraperitoneally, etc.). However, after their inoculation, most of the liposomes are dispersed from the site of inoculation, and either degraded or endocytosed by phagocytic cells such as polymorphonuclear and mononuclear leucocytes, and macrophages (Poste, 1983, Biol. Cell, 47: 19-38). Thus, the release of entrapped drug from liposomes is limited to the period of time between inoculation and degradation or clearance of liposomes from body fluids.
Sustained drug release characteristics can be ascribed to other types of drug microcarriers such as lipid microvesicles (microreservoirs) described by Sears, U.S. Pat. No. 4,298,594.